Lightweight plastics composites comprise a versatile family of relatively low density, cellular materials which are made in a variety of processes. Organic and inorganic fillers are typically added to plastics to modify or improve one or more properties of the plastic, thereby making the plastic composite more suitable for a particular application.
For instance, for the purposes of density and/or for the enhancement of insulation properties, the ideal filler is voids. In the plastics industry, plastics that contain voids (cells) are classified as foams. Depending on the degree of foaming, that is, the volume fraction of the foam making up the voids, the properties of such materials may be remarkably different from the basic material. Foaming agents, more commonly known as blowing agents, are chemicals that can be incorporated into the plastic, that lead to the development of cells through the release of a gas at the appropriate time during processing. The amount and types of blowing agents influences the density of the finished product by its cell structure. In essence, these composites are polymers which have typically been expanded by a gas in volume such that they have a uniformly cellular structure which can look like an extremely fine honeycomb or a mass of very tiny ping-pong balls fused together. The cells of some foamed plastics are large enough to be seen, while cells in others are so fine that a microscope is needed.
The development of a cellular structure in a thermoplastic polymer can be enhanced by the aid of a nucleating agent. Nucleated polymers tend to have a finer and more consistent cell size than unnucleated ones. Coarse cells tend to be more brittle than finer cell structures. Conventional nucleating agents can be roughly classified as inorganic additives, such as talc, silica, koalin, and organic salts of specific acids or pigments.
As well, interest in the plastics industry has been focused upon the use of lower cost organic and inorganic fillers for extending and reinforcing thermoplastics. With respect to the inorganic fillers, an emphasis has been placed on the utilization of fillers such as glass fiber, calcium carbonate, clay, talc, wollastonite, silica and mica. These aforementioned fillers are all inorganic minerals that tend to have relatively high specific gravities. Thus, the resultant composite does not tend to be particularly lightweight. Further, in most cases, these fillers tend to require some pre-treatment in order to be compatible with the thermoplastic polymers they are blended with, thus contributing to a higher cost of the composite material.
Specifically, the common inorganic fillers utilized at present, such as calcium carbonate and glass fibers, require beneficial surface treatment to improve adhesion, lubrication and homogenous mixing with the thermoplastic polymer or resin component. For example, calcium carbonate generally requires a precoating of a coupling agent such as a stearate for blending with rigid Polyvinyl Chloride resins. Further, some applications of flue gas residue fly ash as a filler to thermoplastic Polyvinyl Chloride require the surface treatment of the fly ash with a high cost water system amino-vinyl type silane coupling agent to improve the fillers oil absorption, particle surface pH control and adhesion to the polymer. This is important since the viscosity of filler loaded resins are affected by pH controlled reactions between the filler particle surfaces and the plastic matrix.
In recent times, pulverized coal has become the primary fuel for burning in electric utility steam boilers for the generation of electricity, with hundreds of millions of tons of coal consumed annually. Approximately 10% of the coal burned constitutes inorganic matter which is fused in combustion and carried with the flue (exhaust) gas as the carbon is consumed by oxidation. The flue gas cools as it flows out of the steam generators, and the inorganic matter takes the form of spherical ceramic particles typically in the range of between about 1 to 200 microns. The particles are then captured by electrostatic precipitators and collected in hoppers or disposed of in lagoons. These particles are known as cenospheres and are comprised primarily of a calcinated clay.
Environmental considerations play a key role in the aggressive drive for finding applications for this inorganic residue waste material. Some forms of flue gas residue, such as the fly ash derivative, have been used in various products such as light weight concrete, cement, brick, grout, protective coatings and flowable fill. The cenosphere fraction of the fly ash is about 20% by volume of the total fly ash residue. However, the utilization of the cenosphere form of the flue gas residue has had limited applications, resulting in concerns with respect to the disposal of the cenosphere fraction.
More particularly, the addition of the ceramic cenospheres fraction of flue gas residue fly ash as a filler to thermoplastic and thermoset resinous materials has received little attention by the industry. The primary applications focus upon adding the cenospheres to act as a thickening agent for polyesters, epoxy and other liquid thermosets to enable these composites to become more "putty-like" and more easily spread, such as in autobody repair fillers. Further, more recent research indicates that the use of cenospheres in moldings of polyester and phenolic resins may result in improved or comparable mechanical properties as compared to conventional fillers such as calcium carbonate.
With respect to organic fillers, the supply and cheapness of wood fiber on a world-wide basis, as well as its other advantages, has also made wood fibers a desirable material for filler/reinforcing in combination with thermosetting or thermoplastic materials.
Conventional wood composites comprised of wood fibers and thermosetting polymers, such as plywood and fiberboard, which are offered on the market today tend to suffer from low moisture resistance due to the hydrophilic nature of the wood. For this reason, amongst others, during the last several years, interest has grown in the combination of wood fibers with thermoplastic matrices for the production of composite materials, particularly for low cost/high volume applications. Using a thermoplastic polymer in the composite has been found to result in an improvement in the water resistance and other properties of the composite material.
Further, the development of wood fiber and thermoplastic composites has occurred because wood-derived fillers or reinforcement for thermoplastic polymers may have several further advantages. For instance, the use of wood fibers as filler and reinforcement for thermoplastics may result in a composite material having a lower cost to produce, a lower density, a greater tensile modulus (which results in lower filler damage during processing), and less abrasiveness to processing equipment as compared to other fillers or reinforcing materials on the market for thermoplastics. In addition, wood fiber fillers are derived from a renewable resource.
However, the use of an organic wood fiber filler, and various inorganic fillers, in combination with thermoplastics has been somewhat limited mainly as a result of difficulties experienced in dispersing the filler, which tends to be naturally hydrophilic, in the thermoplastic polymer matrix, which tends to be naturally hydrophobic. In addition, as a result of the hydrophilic nature of the filler and the hydrophobic nature of the thermoplastic, there appears to be a weak interaction therebetween. Insufficient dispersion and weak interactions may result in poor adhesion (wettability and particle-bonding), and in consequence, inferior mechanical properties in the resultant composite materials.
More particularly, because the filler tends to be insoluble in the thermoplastic polymer matrix, a polymer matrix interface is created. The interface/interphase region strongly affects the end properties of the composite material. The interface is the surface between the two phases, whilst the interphase is defined as the region between the filler and the thermoplastic polymer matrix. Poor interaction between the filler and the polymer matrix decreases the interfaces between the phases.
Several approaches have been developed for improving the compatibility of wood fiber fillers and the thermoplastic polymer matrix. More particularly, emphasis has been placed on improving the interfacial adhesion between the hydrophilic wood particles and the hydrophobic thermoplastic polymer matrix. Two primary approaches have been used: (1) modification of the wood surface before processing; and (2) use of a compatibilizing or coupling agent during compounding.
Many different approaches are reported in the literature for surface treatment of the wood, for example, coating and grafting of the wood fibers. The basic method employed is to chemically modify the wood filler component prior to compounding.
With respect to the use of compatibilizing agents, referred to in the industry as coupling agents, it has been determined that the adhesion between the wood fibers and the synthetic polymer can be improved when the coupling agent is added during processing. Coupling agents may act to modify the interface between the wood fibers and the thermoplastic polymer by interacting therewith and forming a link or bond between the components. Thus, the use of coupling agents may enhance the inherently poor attraction between the polar wood and the non-polar thermoplastic component.
The "coupling agents" previously used in the industry in the production of wood fiber-thermoplastic polymer composite materials cover a broad range of complex polymers, such as: maleic anhydride polyethylene (MAPE)/polypropylene (MAPP); styrene-butadiene-styrene; carboxylic wax; ionomer modified polyethylene; and low molecular weight polypropylenes. However, these types of coupling agents may be expensive to use or difficult to disperse within a matrix mixture.
There is therefore a need in the industry for a composite material comprised of relatively low cost components and exhibiting comparable or improved properties as compared to known composites in the market. More particularly, there is a need for an improved composite material comprised of an inorganic filler, or a combination of inorganic and organic fillers, and a thermoplastic polymer There is also a need for a relatively cost effective coupling agent for use in the improved composite material to enhance the properties thereof. Further, there is a need for a relatively lightweight composite material, as compared to known composites. Finally there is also a need for a process for producing the composite material.